For illuminating units (backlight) for use in various liquid crystal displays such as a liquid crystal television set and the like, there are known a sidelight method in which a light source is placed on the side of a liquid crystal display panel and light from the light source is used to illuminate the liquid crystal display panel through a light guiding plate formed of an acrylic resin or the like and a direct method in which a light source is placed on the backside of a liquid crystal display panel.
In recent years, screens of liquid crystal displays are increasing in size. As illuminating units for large-screen liquid crystal displays, there have been increasingly employed direct illuminating units that do not much increase in weight for an increase in size and that can easily attain high luminance.
A liquid crystal display using a direct illuminating unit has a liquid crystal display panel, a plurality of linear light sources for illuminating the liquid crystal display panel, a light refractive plate that is arranged opposite to the light sources through the liquid crystal display panel and that is for reflecting light from the light sources and a light diffusing plate that is arranged between the light sources and the liquid crystal display panel and that is for uniformly diffusing light from the light sources and light reflected from the light reflective plate to the gamut of the liquid crystal display panel, and it has a constitution of the light reflective plate/light sources/light diffusing plate/liquid crystal display panel in this order. In some cases, one or a plurality of optical control films are arranged between the light diffusing plate and the liquid crystal display panel.
As a light refractive plate, there are known a light reflective plate having a metallic mirror surface formed by depositing a metal vapor deposition film on the surface of a substrate such as a metal and the like, a light reflective film formed of a film containing a white pigment such as titanium oxide and the like. Of these, the light reflective plate having a metallic mirror surface has a high reflectance, while it has a problem that since its reflection is specular reflection, reflected light does not go out in the front direction of a display and light cannot be effectively used. In addition thereto, it also has a problem that a so-called “cyclic luminance nonuniformity” is intense, that is, images of a plurality of linear light sources are liable to be maintained as they are, and in its light-emitting surface, regions directly above/on the linear light sources are brighter and any other regions are darker. The light reflective plate formed of a film containing a white pigment is required to have a large content of the pigment for inhibiting the leak of light through its back surface. Since, however, the white pigment absorbs light having a specific wavelength, an increase in light loss is no longer negligible when the content thereof in the film is increased, and the problem is that the reflectance decreases or that reflected light has a yellow tint.
Various reflective plates have been proposed for overcoming the above defects of reflective plates. For example, Japanese Patent No. 2925745 proposes a light reflective plate formed of a thermoplastic polyester foamed sheet. This light reflective plate is formed by impregnating a thermoplastic polyester resin sheet with a high-pressure inert gas and then heating this resin under atmospheric pressure to form bubbles, so that gas bubbles are contained in the resin in place of the white pigment. However, the light reflective plate formed of the above polyester foamed sheet has a defect that no sufficient reflectance can be obtained. Further, JP2005-115051A discloses a reflective plate obtained by forming a light-resistant layer on at least one surface of a polycarbonate resin foamed material layer. In this publication, specifically, a foamed sheet obtained by foaming a block copolymer of polycarbonate and polysiloxane is described as the polycarbonate resin foamed material layer. While this foamed sheet has a high reflectance, it has a problem that gas bubbles are connected in a line to be in the state of being cracked since the foaming during the production thereof is not stabilized, and besides this, the sheet has a defect that it is poor in heat resistance and is also poor in abrasion resistance. There has not yet been known any light reflective plate that has a high reflectance, that can maintain the color tone of light from a light source as it is emitted and that can inhibit the cyclic brightness nonuniformity caused due to the use of a plurality of linear light sources.
On the other hand, as a light diffusing plate, for example, JP-A 03-078701 discloses a resin composition prepared by adding calcium carbonate and titanium oxide to a polycarbonate resin, JP-A 05-257002 discloses a resin composition prepared by adding calcium carbonate or a crosslinked polyacrylate resin to a polycarbonate resin, JP-A 08-188709 discloses a resin composition prepared by incorporating a beady crosslinked acrylic resin into a polycarbonate resin, and further, JP-A 09-20860 discloses a resin composition prepared by adding a beady crosslinked acrylic resin and a fluorescent brightener to a polycarbonate resin. These techniques are those in which light diffusing agents are incorporated into resins for realizing the capability of light diffusing. However, there is involved a problem that the light diffusing agents per se are altered, or the yellowing of the matrix resins is induced, due to a thermal history that they suffer during the preparation of the light diffusing plates. JP-A 2002-116306 discloses a light diffusing sheet formed of a synthetic resin containing fine cells dispersed therein, and it shows that such a light diffusing sheet is used as an optical sheet in a backlight unit for a liquid crystal display. However, when this light diffusing sheet is used as a light diffusing plate for the direct backlight, there are involved problems that the light diffusing capability is insufficient and that light sources are easily seen through it so that the above cyclic brightness nonuniformity cannot be sufficiently inhibited. There has not yet been known any light diffusing plate that has desired performances in both transmittance and light diffusing capability.
Further, light reflective plates and light diffusing plates for use in liquid crystal displays of which the screen sizes are increasing as described above are required to have uniform performances all over their large areas. However, there has not yet been known any light reflective plate or light diffusing plate that exhibits uniform and excellent performances with regard to the capability of diffuse reflection of visible light or with regard to transmittance and the capability of diffusion of visible light. Further, it is considered that a form prepared by winding a sheet having a width of approximately 1 to 2 m and a length of approximately 100 to 1,000 m in the form of a roll is advantageous as a form for the production and shipment of the light reflective plate from the viewpoint of a production cost and a transportation cost. However, there has not yet been known any light reflective plate that exhibits uniform and excellent performances all over such a large area with regard to the capability of diffuse reflection of visible light.